Unlike existing networks, wireless sensor networks are basically configured to collect information automatically and remotely rather than to perform communication, and widely used in a variety of applications, such as scientific, medical, military, and commercial uses.
Each sensor network includes a plurality of sensor nodes configured to sense and transmit information using sensors and a sink node configured to transmit the information collected by the plurality of sensor nodes to the outside.
In this case, each sensor node has a simple configuration including a sensor configured to sense information, a processor configured to process the sensed information, and a radio transceiver configured to transmit the processed information. For the convenience of installation and use, each sensor node is required to have little power consumption and miniaturization so as to operate for the long time using limited battery power. To satisfy the requirement of low power consumption and miniaturization, the sensor node generally has a simplified function and structure, having limited calculation capabilities.
In order to configure an antenna detecting a propagation direction or indicating a directional radiation direction, a phased array antenna or a wireless communication system supporting multiple ports has generally been employed. However, the phased array antenna and the wireless communication system need high power consumption and high calculation capability and need to include a plurality of RF ports.
In comparison, the small wireless device having a low-power consumption and limited calculation capabilities like the sensor node mostly supports an RF output of a single port and needs to control only the direction of a limited number of sectors.
Thus, in the case of the small wireless device such as the sensor node, it does not use a plurality of radiators like the phased array antenna to control the propagation direction but to configure an antenna with a single radiator and a plurality of parasitic elements to adjust an electrical length of a parasitic element and uses the interference between elements.
FIG. 1 illustrates a small sector antenna generally used in a small wireless device.
Referring to FIG. 1, the small sector antenna 10 includes a single monopole antenna 11 and a plurality of parasitic elements 12 installed at regular intervals in a circular form around the monopole antenna 11. A reference numeral 13 in FIG. 1 denotes a disk-type metal ground on which the monopole antenna 11 and the plurality of parasitic elements 12 are fixed. In this case, the length of the monopole antenna 11, the length of the parasitic elements 12, the distance between the monopole antenna 11 and the parasitic element 12, and the thickness of the disk-type metal ground are designed to be λ/4 (λ: radio signal wavelength).
The radiation direction of the small sector antenna is controlled by changing an electrical equivalent length according to change in capacitance of the plurality of parasitic elements 12.
In the related art, a varactor diode is commonly used as means of controlling the capacitance of the plurality of parasitic elements 12, which, however, disadvantageously accompanies complicated calculation to appropriately adjust a nonlinear corresponding relationship between a bias voltage and the radiation direction.
In addition, in order to convert a digital bit string generated due to the complicated calculation into a bias voltage of the varactor diode, a digital-to-analog converter (DAC) must be provided to increase the power consumption.
FIG. 2 is a schematic block diagram showing a radiation direction control device for a small sector antenna according to a conventional technology.
Referring to FIG. 2, the radiation direction control device for a small sector antenna includes a plurality of capacitance blocks 31, a plurality of switching units 32, and a control unit 33. The plurality of capacitance blocks 31 and the plurality of switching units 32 are matched to a plurality of parasitic elements 12 in a one-to-one fashion, and connected in series between the parasitic elements 12 and a ground. The control unit 33 controls the switching operations of the plurality of switching units 32.
Each of the switching units 32 includes an n+2 number of selective contact points T1 to Tn+2, two more than the number (n) of the capacitors of each of the capacitance blocks 31, where the n number of contact points T1 to Tn are connected to the capacitors of the capacitance block 31, and the two remaining contact points Tn+1 and Tn+2 are directly connected with the corresponding parasitic elements 12 and set for a short mode and an open mode. A fixed contact point T0 of the switching unit 32 is connected to a ground. The switching unit 32 is configured to connect one of the plurality of capacitors to the corresponding parasitic element 12 or short-circuit or open the parasitic element 12 by selectively connecting one of the plurality of selective contact points T1 to Tn+2 to the fixed contact point T0 according to the control of the control unit 33.
After selecting one of the selective contact points T1 to Tn+2 according to a control signal of the control unit 33, the switching unit 32 preferably maintains the selection until a next control signal is applied, for which the switching unit 32 may be implemented as a digital switch.
Each of the plurality of capacitance blocks 31 includes a plurality of capacitors C1 to Cn, each having a different capacitance value, connected in parallel. The one ends of the plurality of capacitors C1 to Cn are commonly connected to the parasitic element 12 and the other ends of the plurality of capacitors C1 to Cn are connected with the plurality of selective contact points T1 to Tn provided in the switching unit 32.
When the small sector antenna including the single RF port and the plurality of parasitic elements and the radiation direction control device for the small sector antenna are arranged in the same plane, the area of the antenna is increased in proportion to the length of the operating wavelength. Accordingly, there is a limitation in that if frequency becomes lower, the wavelength becomes longer and thus the area of the antenna is increased.
In addition, generally, the low power wireless device has a common ground, which is needed for the stable operation of the antenna, arranged in a substrate on which an RF element, a modem, etc. are mounted. If the common ground is not distributed appropriately to the radiation, the antenna has a ground included therein. In this case, the needed size or area of the ground is proportional to the wavelength, which may operate as a limitation in reduction of the size of a switchable directional control antenna. The low-power wireless transmitters such as an IoT transmission device, a sensor node, etc. are miniaturized due to the development of integration technologies. However, actually, research is not conducted so much about the miniaturization of the directional control antenna.